RF transmitters are known that use complex modulators, which convert baseband, i.e., zero IF (intermediate frequency), signals, consisting of I (in-phase) and Q (quadrature) components, to RF signals using spectrally efficient modulation techniques such as QPSK (quadrature phase-shift keying), QAM (quadrature amplitude modulation), and so forth. To up convert the zero IF I and Q signals to an intermediate frequency, the prior art uses a VCO (voltage-controlled oscillator) to produce an offset oscillator signal at the same frequency. A significant problem that arises with this arrangement is the coupling of the modulated IF signal into the VCO, causing unwanted remodulation of the VCO and subsequent spectral spreading of desired IF signal. This coupling is accentuated by the extremely high loop gain of the VCO itself in the vicinity of its oscillation frequency.
In applications where modulation with fast spectral roll-off characteristics is used to increase spectral efficiency, remodulation levels below -60 dB are required. The problem is further accentuated in other designs where, in order to simplify the transmitter, it is desirable to perform direct conversion from zero IF to the transmit frequency. Direct conversion would eliminate the need for an offset synthesizer, an image filter, and other components using a modulator local oscillator injection at the transmitter output frequency. It is impractical to use shielding, particularly in portable equipment, to sufficiently reduce the coupling from the high powered modulated PA signal to a VCO operating at the same frequency such that the modulation is acceptable.
Accordingly, there is a need for an apparatus that reduces or eliminates remodulation such that fast spectral roll-off is achievable for a direct conversion or other transmitter.